Various fluid systems include a pressure fluid supply which is to be divided between two output systems in accordance with flow changes in one of the output systems. For example, steam generating plants generally include a centrifugal feed water pump which provides an output flow to a feed water heater. In these plants, it is usual to operate the pump continuously and to utilize a check valve whose position is responsive to the rate of flow from the pump to the heater in accordance with the demand flow requirements of the heater. Flow through the pump also functions to cool the pump. Therefore, during periods of relatively low demand from the heater, when the output of the pump would fall below a minimum flow rate, it is possible for the pump to overheat. In order to protect against the pump overheating when the demand from the heater falls below the minimum flow rate, it is common practice to maintain the output of the pump at the minimum flow rate and recirculate the excess flow back to the pump. By recirculating a portion of the output, the pump is protected from overheating.
Recirculation of the output from the pump can be effected a number of ways. One is by manual operation of a bypass valve. This, however, is relatively expensive and unreliable. Another approach is to employ a bypass valve controlled by a flow meter which detects when the flow on the inlet side of the pump drops below the minimum rate required to cool the pump. When this condition is sensed, the flow meter causes the bypass valve to be opened maintaining the pump output and diverting that portion of the flow not required by the feed water to a sump connected to the inlet of the pump. Such an arrangement involves relatively complex and expensive apparatus and has not been found to be satisfactory.
Yet another approach to recirculating the output from the pump involves the use of a modulating flow control valve assembly including a main check valve whose position is responsive to the rate of flow from the pump to the heater and a bypass valve for recirculating flow back through the pump during periods of low demand by the heater. An example of this type of valve assembly is shown in U.S. Pat. No. 4,019,527. While modulating flow control valve assemblies have been found to be generally satisfactory for most systems, those that are known at the present time are not entirely satisfactory in certain applications which have the concurrent requirements of effective operation with minimum pressure drop, relatively simple construction, low cost, and easy adjustment, especially in the field, of the bypass starting position and capacity.